Well, here's a random idea to slow it down - the second from the end reform does not vassalise the entire empire, but enables you to spend 5/10 authority a pop to vassalise each via the diplomacy screen, or maybe via a magistrate. They can, of course, refuse, and it would be a war of your agression, to draw in all of their allies. Then, at any point, for the cost of some 50 IA, one could unite all of their vassals and themselves into the HRE, with 1 decentralization slider move per state vassalized and -1 stab, because I'm guessing that kind of a unification, even if one has been preparing it for years, would still leave quite a bit of chaos in the empire.